英文:
Golang struct array values not appending In loop
问题
这是我的代码：
package main
import(
"fmt"
)
type Category struct {
Id int
Name string
}
type Book struct {
Id int
Name string
Categories []Category
}
func main() {
var book Book
book.Id = 1
book.Name = "Vanaraj"
for i := 0; i < 10; i++ {
book.Categories = append(book.Categories, Category{
Id : 10,
Name : "Vanaraj",
})
}
fmt.Println(book)
}
我需要将值追加到categories中。这些值只追加了一次，但我需要将值追加到数组中。
如何修复这个问题？
英文:
This is my code:
package main
import(
"fmt"
)
type Category struct {
Id int
Name string
}
type Book struct {
Id int
Name string
Categories []Category
}
func main() {
var book Book
book.Id = 1
book.Name = "Vanaraj"
for i := 0; i < 10; i++ {
book.Categories = []Category{
{
Id : 10,
Name : "Vanaraj",
},
}
}
fmt.Println(book)
}
I need to append the values to the categories. The values are appending only one time. But I need to append the values to the array.
How to fix this?
答案1
得分: 18
你在每次循环迭代中都没有向book.Categories添加任何内容，你总是创建一个新的切片，并将其赋值给book.Categories。
如果你想要添加值，可以使用内置的append()函数：
for i := 0; i < 10; i++ {
book.Categories = append(book.Categories, Category{
Id: 10,
Name: "Vanaraj",
})
}
输出结果（在Go Playground上尝试）：
{1 Vanaraj [{10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj}]}
另外请注意，如果你事先知道迭代次数（在你的情况下是10），你可以事先创建一个足够大的切片，然后使用for ... range，只需将值分配给正确的元素，而无需调用append()。这样更高效：
book.Categories = make([]Category, 10)
for i := range book.Categories {
book.Categories[i] = Category{
Id: 10,
Name: "Vanaraj",
}
}
英文:
You are not appending anything to book.Categories, in each iteration of the for loop you always create a new slice with a composite literal and you assign it to book.Categories.
If you want to append values, use the builtin append() function:
for i := 0; i < 10; i++ {
book.Categories = append(book.Categories, Category{
Id: 10,
Name: "Vanaraj",
})
}
Output (try it on the Go Playground):
{1 Vanaraj [{10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj} {10 Vanaraj}]}
Also note that if you know the iteration count in advance (10 in your case), you can create a big-enough slice beforehand, you can use for ... range and just assign values to the proper element without calling append(). This is more efficient:
book.Categories = make([]Category, 10)
for i := range book.Categories {
book.Categories[i] = Category{
Id: 10,
Name: "Vanaraj",
}
}
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